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The C standardStandard, section 3.4.3 [ISO/IEC 9899:2011], defines undefined behavior as

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Behavior can be classified as undefined by the C standards committee for the following reasons:

• to To give the implementor license not to catch certain program errors that are difficult to diagnose
• to To avoid defining obscure corner cases which would favor one implementation strategy over another
• to To identify areas of possible conforming language extension: the implementor may augment the language by providing a definition of the officially undefined behavior

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Increasingly, compiler writers are taking advantage of undefined behaviors in the C programming languages to improve optimizations. Frequently, these optimizations are interfering These optimizations frequently interfere with the ability of developers to perform cause-effect analysis on their source code, that is, analyzing to analyze the dependence of downstream results on prior results.   Consequently, these optimizations are eliminating causality in software and are increasing the probability of software faults, defects, and vulnerabilities.

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#include <assert.h>

int foo(int a) {
  assert(a + 100 > a);
  printf("%d %d\n", a + 100, a);
  return a;
}

int main(void) {
  foo(100);
  foo(INT_MAX);
}

This code tests checks for signed integer overflow by testing to see if whether a + 100 > a. This test cannot evaluate to false unless an integer overflow occurs. However, because a conforming implementation is not required to generate code for undefined behavior, and signed integer overflow is undefined behavior, this code may be compiled out. For example, GCC version 4.1.1 optimizes out the assertion for all optimization levels, and version 4.2.3 optimizes out the assertion for programs compiled with -O2-level optimization and higher.

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Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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Bibliography

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ISO/IEC TR 24772 "BQF Unspecified behaviour," "EWF Undefined behaviour," and "FAB Implementation-defined behaviour"

Sources

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